The present invention relates to helical screw type compressors. More specifically, the present invention relates to a multi-screw compressor having, e.g., a male rotor and at least two female rotors.
Helical type compressors are well known in the art. One such helical compressor employs one male rotor axially aligned with and in communication with one female rotor. The pitch diameter of the female rotor is greater than the pitch diameter of the male rotor. Typically, the male rotor is the drive rotor, however compressors have been built with the female rotor being the drive rotor. The combination of one male rotor and one female rotor in a compressor is commonly referred to as a twin screw or rotor, such is well know in the art and has been in commercial use for decades. An example of one such twin rotor commonly employed with compressors in the HVAC (heating, ventilation and air conditioning) industry is shown in FIG. 1 herein, labeled prior art. Referring to FIG. 1 herein, a cross sectional view of a male rotor 10 which drives an axially aligned female rotor 12 is shown. Male rotor 10 is driven by a motor, not shown, as is well known. Male rotor 10 has four lobes 14-17 with a 300.degree. wrap and female rotor 12 has six flutes 18-23 with a 200.degree. wrap. The resulting gap between the male and female rotors requires oil to be introduced into the compression area for sealing, however, the oil also provides cooling and lubricating, as is well known. However, the introduction of this oil requires the use of an oil separation device, to separate the oil from the refrigerant being compressed in HVAC compressors. The primary benefit of the twin rotor configuration is the low interface velocity between the male and female rotors during operation. However, the twin rotor configuration is not balanced and therefore incurs large radial bearing loads and thrust loads. The obvious solution to alleviating the bearing load problem would be to install sufficiently sized bearings. This is not a feasible solution, since the relative diameters of the rotors in practice result in the rotors being too close together to allow installation of sufficiently sized bearings.
The prior art has addressed this problem, with the introduction of compressors employing `so-called` single screw technology. Referring to FIGS. 2 and 3 herein, labeled prior art, a drive rotor 24 with two opposing axially perpendicular gate rotors 26 and 28 is shown. Rotor 24 is driven by a motor, not shown, as is well known. Rotor 24 has six grooves 30 and each gate rotor 26, 28 has eleven teeth 32, 34, respectively, which intermesh with grooves 30. The gate rotors 26 and 28 are generally comprised of a composite material which allows positioning of the gate rotor at a small clearance from the drive rotor. This clearance is small enough that the liquid refrigerant itself provides sufficient sealing, the liquid refrigerant also provides cooling and lubrication. The rearward positioning of gate rotors 26 and 28 and the positioning on opposing sides of drive rotor 24, (1) allows equalizing suction of pressure at both ends of rotor 24 thereby virtually eliminating the thrust loads encountered with the above described twin screw system and (2) balances the radial loading on rotor 24 thereby minimizing radial bearing loads. However, the interface velocity between the gate rotors and the drive rotor are very high. Accordingly, a common problem with this system is the extensive damage suffered by the rotors when lubrication is lost, due to the high interface velocities of the rotors.
One method of overcoming these deficiencies of the prior art is presented in U.S. Pat. No. 5,642,992 commonly assigned to this application. The compressor in '992 includes a male rotor which is axially aligned with and in communication with at least two female rotors. The male rotor is driven by a motor, in other words the male rotor is the drive rotor. The male rotor has a plurality of lobes which intermesh with a plurality of flutes on each of the female rotors. The pitch diameters of the female rotors are now less than the pitch diameter of the male rotor.
The male rotor comprises an inner cylindrical metal shaft with an outer composite material ring mounted thereon. The ring includes the lobes of the male rotor integrally depending therefrom. The lobes of the male rotor being comprised of a composite material allows positioning of the female rotors at a small clearance from the male drive rotor. This clearance is small enough that the liquid refrigerant itself provides sufficient sealing, however, the liquid refrigerant also provides cooling and lubrication.
The positioning of the female rotors on opposing sides of the male rotor balances the radial loading on the male rotor thereby minimizing radial bearing loads. Further, due to a larger diameter male drive rotor as compared to the male drive rotor in the prior art twin screw compressors, and therefore, additional distance between the rotors, any female radial bearing stress can be further minimized with sufficiently sized bearings. It will also be appreciated, that interface velocity between the male and female rotors during operation is very low, whereby the extensive damage suffered by the prior a rt single screw compressors when lubrication is lost, due to the high interface velocities of the rotors, is reduced.
The compressor includes a housing having an inlet housing portion, a main housing portion and a discharge housing portion. An induction side plate and a discharge side plate are mounted on the male rotor. The outside diameter of the induction side plate is equal to the root diameter of the male rotor. The outside diameter of the discharge plate is equal to the crest diameter of the male rotor. These plates serve two purposes, to secure the male rotor components and to equalize suction pressure at both ends of the male rotor, thereby virtually eliminating the thrust loads encountered with the prior art twin screw compressors. It will be appreciated that the discharge plate blocks the axial port area of the male rotor which results in a reduction in overall discharge port area.